1. Field of the Invention
The present invention relates to a conductive material for wiring lines and electrodes of an active matrix-type liquid crystal display (LCD) apparatus and a gate electrode of a metal oxide semiconductor (MOS) device.
2. Description of the Related Art
Active matrix-type LCD apparatuses are so thin and light that they are often used in various display apparatuses. Also, since the active matrix-type LCD apparatuses can be of a large size and fine screen with a high quality and a low power dissipation, cathode ray tubes (CRTs) have been replaced with the active matrix-type LCD apparatuses.
In an active matrix-type LCD apparatus, one thin film transistor (TFT) is provided as a switching element for each pixel. The TFT is constructed by a gate electrode, a semiconductor active layer made of amorphous silicon opposing the gate electrode, a drain electrode, and a source electrode connected to a transparent pixel electrode.
In the active matrix-type LCD apparatus, in order to effectively increase the area of the screen and make the screen fine, gate bus lines (scan bus lines) and drain bus lines (signal bus lines) connected to TFTs need to be narrower and longer.
For example, in an inverted staggered TFT where the gate electrode is located beneath the semiconductor active layer, the scan bus line connected to the gate electrode of the TFT and the gate electrode need to be thin and low in resistivity. In addition, the scan bus line and the gate electrode need to be chemical-resistant in processes such as photolithography processes and rinsing processes.
Also, in a staggered TFT where the gate electrode is located above the semiconductor active layer, the signal line connected to the drain electrode of the TFT and the gate electrode need to be easily formed and low in resistivity. In addition, the signal bus line and the drain electrode need to be chemical-resistant in chemical processes such as photolithography processes and rinsing processes.
A first prior art conductive material for the scan line, the signal line, the gate electrode and the drain electrode is made of metal such as tantalum (Ta), titanium (Ti), chrome (Cr) or molybdenum-tantalum alloy (Mo--Ta).
The first prior art conductive material is, however, high in resistivity, not easy to be formed, and of low chemical-resistance of in chemical processes.
A second prior art conductive material for the above mentioned lines and electrodes is made of aluminum (Al) which is very low in resistivity, for example, 3 .mu..OMEGA..multidot.cm.
The second prior art conductive material is, however, subject to hillocks and voids, thus creating short-circuits and disconnections in the conductive material. Also, aluminum is melted into chemicals during chemical processes, so that the chemicals are contaminated by the melted aluminum.
In order to suppress the generation of hillocks and voids, a third prior art conductive material for the above-mentioned lines and electrodes is made of aluminum alloy including about 0.1 to 10 atm percent of at least one of Ta, Ti and zirconium (Zr) (see JP-A-5-100248). This conductive material is heated at 250 to 500.degree. C. Also, this conductive material is coated by Ta which is chemical-resistant in chemical processes, or is anodized, thus suppressing the generation of hillocks and voids. In this case, the resistivity is about 10 .mu..OMEGA..multidot.cm.
However, the third conductive material has complex manufacturing steps such as a Ta coating step or an anodizing step, thus increasing the manufacturing cost. Also, aluminum is melted into chemicals during chemical processes, so that the chemicals are contaminated by the melted aluminum.
A fourth prior art conductive material for the above-mentioned lines and electrodes is made of Mo or tungsten (W). This conductive material is relatively low in resistivity, for example, 15 .mu..OMEGA..multidot.cm. Also, this conductive material can be formed by using both a wet etching process and a dry etching process in the same way as with aluminum.
In the fourth prior art conductive material, however, Mo and W are not chemical-resistant in chemical processes. Also, W has a bad substrate contact characteristic.
A fifth prior art conductive material for the above-mentioned lines and electrodes is made of Mo--W alloy (see M. Ikeda, "TFT-LCD Gate and Data Bus-Line Design and Process Technologies", SID 95 Digest, pp.11-14, 1995). This conductive material is also relatively low in resistivity, for example, 15 .mu..OMEGA..multidot.cm. Also, this conductive material can be easily formed by adjusting the composition of Mo and W.
The fifth prior art conductive material is, however, still low in chemical-resistance.
A sixth prior art conductive material for the above-mentioned lines and electrodes is made of Ta alloy which includes 30 to 80 atm percent of Ta (see JP-A-3-34368). The Ta alloy is a composite metal of Ta and W, Ta and nickel (Ni), Ta and cobalt (Co), Ta and rhodium (Rh), or Ta and iridium (Ir). Particularly, when the atm percent of Ta is 50 to 70, the resistivity is low, i.e., 30 to 60 .mu..OMEGA..multidot.cm. Also, this conductive material can be formed by an RF sputtering process using a W (or Ni, Co, Rh, Ir) target and a Ta target whose sputtered areas are adjusted.
The sixth prior art conductive material is, however, still high in resistivity, as compared with .alpha.-Ta (25 to 30 .mu..OMEGA..multidot.cm) or Cr (25 .mu..OMEGA..multidot.cm). Thus, the sixth conductive material is not suitable for large size fine LCD apparatuses.
A seventh prior art conductive material for the above-mentioned lines and electrodes is made of W/Ta alloy which includes 30 to 95 atm percent of Ta (see JP-A-1-275753). Particularly, when the atm percent of Ta is 50 to 60, the resistivity is low, i.e., 35 to 50 .mu..OMEGA..multidot.cm. Also, this conductive material can be easily formed by a dry etching process. Further, this conductive material is resistant to acids such as sulfating water. In addition, an anodized oxide layer can be easily formed on this conductive material.
The seventh prior art conductive material is, however, still high in resistivity, as compared with .alpha.-Ta (25 to 30 .mu..OMEGA..multidot.cm) or Cr (25 .mu..OMEGA..multidot.cm). Thus, the seventh conductive material is not suitable for large size fine LCD apparatuses.
Also, in the sixth and seventh prior art materials, since the etching selectivity by a wet etching process using fluorine acid or fluoric nitric acid is inferior, a dry etching process has to be adopted, thus increasing the manufacturing cost.